1. Field of the Invention
The present invention relates to a motor rotor which can correspond to a circumferential speed-up of an electric motor, and more particularly to a motor rotor which is preferable for an electric motor mounted on a motor-driven supercharger.
2. Description of Related Art
In order to improve an internal combustion engine, there has been widely used a supercharger (called also as “turbocharger”) driven by an exhaust gas of an internal combustion engine and compressing an intake air so as to supercharge. Further, there has been used a supercharger in which an electric motor is installed coaxially with a rotating shaft of a supercharger and an acceleration response or the like is improved by accelerating and assisting a rotational drive of a compressor. The supercharger having a motor-driven assist function by the electric motor as mentioned above is called as a motor-driven supercharger.
A description will be given briefly of a structure of this kind of motor-driven supercharger. A supercharger rotor structured such that a turbine impeller and a compressor impeller are coupled to both ends of a rotating shaft is rotatably supported within a housing. An electric motor is incorporated in the housing. A rotor (a motor rotor) of the electric motor is fixed coaxially with the rotating shaft, and a stator (a motor stator) of the electric motor is arranged in the periphery of the rotor in an inner portion of the housing. If an exhaust gas from the internal combustion engine is supplied to the turbine impeller, the turbine impeller is rotationally driven, and the compressor impeller coupled to the turbine impeller is rotationally driven, thereby compressing an intake air so as to supply to the internal combustion engine. Further, at this time, the rotational drive of the compressor impeller is assisted by the electric motor.
The motor rotor of the motor-driven supercharger mentioned above is disclosed in the following Patent Document 1. FIG. 1 is a cross sectional view showing a conventional motor rotor disclosed in the Patent document 1. The motor rotor is constituted by an inner sleeve 51 inserted and attached to a turbine shaft 50 of the supercharger, a permanent magnet 52 surrounding the inner sleeve 51 around an axis, and a hollow cylindrical outer sleeve 53 surrounding the permanent magnet 52 around the axis. The outer sleeve 53 is fitted in accordance with a shrink fitting in such a manner that it is possible to sufficiently hold the permanent magnet 52 even under a condition that a large centrifugal force acts at a maximum rotational speed of the rotor.
In a manufacturing step of the motor-driven supercharger, a correction of a rotation balance is executed by executing a rotation balance test after assembling the motor rotor. In the case of the conventional motor rotor shown in FIG. 1, the balance correction is executed by scraping a part (a portion shown by a reference symbol A in the drawing) of an end surface of the permanent magnet 52. However, if the permanent magnet 52 is scraped, a magnetic force of the permanent magnet is changed. Since an individual difference exists in the rotation balance correction amount, the magnetic force of the permanent magnet is dispersed per a product in correspondence to a scraped amount. Further, since a crack is generated and a stress becomes uneven by cutting the permanent magnet 52, strength is lowered.
In order to cope with the problem mentioned above, there has been proposed the other conventional motor rotor as shown in FIG. 2. The motor rotor is constituted by an inner sleeve 51 inserted and attached to a turbine shaft 50, a permanent magnet 52 surrounding the inner sleeve 51 around the axis, a pair of end rings 54 and 54 sandwiching the permanent magnet 52 from both sides in an axial direction, and a hollow cylindrical outer sleeve 53 surrounding the permanent magnet 52 and a pair of end rings 54 and 54 around the axis. The outer sleeve 53 is fitted to the permanent magnet 52 and the end rings 54 and 54 in accordance with a shrink fitting.
In the motor rotor having the structure mentioned above, a rotation balance correction is executed by scraping a part (a portion shown by reference symbol B in the drawing) of the end ring 54. In this case, since it is not necessary to scrape the permanent magnet 52, there is not generated a problem that the magnetic force change or the strength reduction is generated.
Patent document 1 corresponds to U.S. Pat. No. 6,085,527 (FIG. 5) discussed above.
An upper side graph in the FIG. 2 showing a distribution in an axial direction of a circumferential stress applied to the outer sleeve 53. In FIG. 2, a horizontal axis corresponds to a position in the axial direction, a vertical axis corresponds to a circumferential stress, and the distribution of a circumferential stress applied to the outer sleeve 53 makes a curve as denoted reference symbol L.
In the motor rotor shown in FIG. 2, since the end ring 54 is constituted by a material that can maintain a sufficient strength even if it is scraped at a time of correcting the rotation balance, a longitudinal elastic modulus of the end ring 54 is larger than a longitudinal elastic modulus of the permanent magnet 52. In other words, the permanent magnet 52 is comparatively softer than the end ring 54 so as to be easily deformed elastically.
Accordingly, as shown in FIG. 2, in a state in which the outer sleeve 53 is fitted to the permanent magnet 52 and the end ring 54, the circumferential stress of a portion in the outer sleeve 53 brought into contact with the end ring 54 becomes larger than that of a portion brought into contact with the permanent magnet 52. Therefore, in the outer sleeve 53, an engagement with the permanent magnet 52 becomes relatively weaker than an engagement with the end ring 54. Since further high circumferential speed is required in the electric motor in recent years, it is necessary to set the engagement between the outer sleeve 53, and the permanent magnet 52 and the end ring 54 stronger.
It is possible to set the strength of the engagement of the outer sleeve 53 in such a manner that a desired fastening force can be obtained with respect to the permanent magnet, however, if the engagement is set too strong so as to correspond to the high circumferential speed, there is a risk that both end portions of the outer sleeve 53 are plastically deformed. Accordingly, in the conventional motor rotor, there is a problem that it is hard to correspond to the further high circumferential speed.